In an optical communication system practically used at present, a repeater is inserted at fixed distance intervals, so as to compensate the attenuation of an optical signal due to a loss in an optical fiber. The repeater is constructed in such a manner that the optical signal is converted into an electrical signal by a photodiode, followed by amplification of the electrical signal by means of an electronic amplifier, and thereafter the electrical signal thus amplified is converted into an optical signal by means of a semiconductor laser or the like, followed by returning of the optical signal to an optical transmission line. If the optical signal can be directly amplified with a low noise as it stands, the optical repeater can be made compact and economized.
In this circumstance, many researches in an optical amplifier capable of directly amplifying an optical signal have been greatly developed. The optical amplifier subjected to the researches is generally classified into (a) an optical fiber amplifier employing, in combination, an optical fiber doped with a rare earth element (Er, Nb, Yb, etc.) and a pumping light; (b) an optical amplifer employing a semiconductor laser doped with the rare earth element; and (c) an optical amplifier utilizing a nonlinear effect in the optical fiber.
Above all, the optical fiber amplifier employing the combination of the rare earth element doped fiber and the pumping light as mentioned in the above type (a) has excellent features such as no polarization dependency, low noise, and small coupling loss to a transmission line. Accordingly, the optical amplifier of this type is expected to remarkably increase a repeating distance in an optical fiber transmission system, and it is also expected to enable multiple distributions of the optical signal.
The principle of the optical amplification achieved by means of a rare earth doped fiber will be described below. If a pumping light beam is introduced into an optical fiber having its core doped with erbium, Er atoms are excited to a higher energy level. Then, if a signal light beam is allowed to impinge on the Er atoms in the optical fiber excited to the high level, the Er atoms undergo a transition to a lower energy level and stimulated emission of radiation occurs. Then, power of the signal light progressively increases as it propagates through the optical fiber and thus amplification of the signal light is achieved.
Optical fiber amplifiers operating on the above described principle are extensively developed. A conventional optical fiber amplifier is structured as described below. That is, a signal light beam is introduced into an Er doped optical fiber through an optical isolator, and at the same time, a pumping light beam emitted from a pumping light source is introduced therein through an optical isolator and a wavelength multiplexer/demultiplexer. By making light power of the pumping light sufficiently great, Er atoms within the Er doped optical fiber can be excited to a higher energy level, so that, by the introduced signal light, stimulated emission of light with the same wavelength takes place and an amplified signal light beam is emitted through the wavelength multiplexer/demultiplexer and an optical isolator to the transmission line.
The optical amplifying action performed by the Er doped optical fiber will be described with reference to the energy level diagram of FIG. 1. Er atoms are raised from the ground level (.sup.4 I.sub.15/2) to the excited level by optical energy of the pumping light with the wavelength 0.98 .mu.m or 1.48 .mu.m and undergo transition within the excited level and fall to the level of 1.55 .mu.m band (.sup.4 I.sub.13/2). At this time, if a light beam of the wavelength 1.536 .mu.m is introduced as the signal light, stimulated emission of the Er atoms staying at the 1.55 .mu.m band level takes place as indicated by the arrow "A" and the signal light is thereby amplified.
At the same time, light due to spontaneous emission with wavelengths 1.53 to 1.57 .mu.m is generated as indicated by the arrow "B" on account of expansion of the 1.55 .mu.m band level. However, the light generated by spontaneous emission in the Er doped optical fiber is amplified within the Er doped optical fiber by the energy of the pumping light as with the signal light, and thereby, the amplification of the signal light is adversely affected and the S/N ratio of the signal light is deteriorated.